Method and apparatus for providing an initial bias and enable signal for a power converter

ABSTRACT

The invention employs a coreless isolated transformer, with associated electronic circuitry, for providing an initial bias and enable signal for control and drive circuitry that is referenced to the output of a converter. The improvement is accomplished by embedding the transformer primary and secondary windings into a multi-layer PCB so that the transformer does not occupy space on the top and bottom surfaces of the PCB The initial bias voltage is used to initialize operation of the control circuit when referenced to the output side of the converter. Thus, complete regulation and drive signals are generated on the output side.

CROSS REFERENCE TO RELATED APPLICATION

This is a non-provisional application based on provisional applicationSer. No. 60/272,551, filed Mar. 1, 2001.

BACKGROUND

1. Field of Invention

This invention generally concerns isolated converter circuitry and moreparticularly relates to means for providing an initial bias and anenable signal for the control circuit referenced to the output ofconverter.

2. Background Discussion

It is a common problem in isolated converters to provide a proper biasfor both primary and output circuitry, particularly during start-up orrestart of the converter. Usually a controller (pulse width modulated(PWM) is one example) is on the input side and the feedback signal isprovided via an opto-coupler, while synchronous rectifiers areself-driven from the transformer windings. There are two drawbacks inusing this approach. First, the use of an opto-coupler generally limitsthe bandwidth of the regulation loop and the maximum ambient temperatureand temperature of the printed circuit board (PCB) to less than about85° C. Secondly, the self-driven synchronous approach is generally not agood solution for higher frequencies.

In addition, protection such as over-voltage protection (OVP) has to beon the output side, which may require an additional opto-isolator justfor over-voltage protection. Therefore, there is an advantage to havingthe control circuit on the output side. The major problem is to providethe necessary initial bias voltage before the converter is started. Onepossible solution is to have a separate isolated converter that willprovide the bias voltage. Such a solution would require an additionalmagnetic core and, if realized employing planar magnetics, would consumea lot of board space.

SUMMARY OF THE INVENTION

The present invention provides a solution to the above problems. Theapparatus of the invention employs a coreless isolated transformer, withassociated electronic circuitry, for providing initial bias and enablesignal for the control and drive circuitry referenced to the output of aconverter. The improvement is accomplished by embedding the transformerprimary and secondary windings into a multi-layer PCB so that thetransformer does not occupy space on the top and bottom surfaces of thePCB The initial bias voltage is needed to initialize operation of thecontrol circuit when referenced to the output side of the converter.Thus, complete regulation and drive signals are generated on the outputside.

A coreless transformer does not use any magnetic core as do typicaltransformers. It is, therefore, important to provide, as best aspossible, coupling between the primary and secondary windings withproper geometry and stack-up on the PCB. Magnetic coupling is throughair so this structure will have small magnetizing inductance and largeleakage inductance. The former imposes a limitation on volt-seconds thatcan be applied across the windings of the transformer, while the latterrequires a proper turns ratio that would compensate for leakageinductance. In addition, by proper geometry (construction) of thewindings of the coreless transformer, as well as stack-up of the PCB,leakage inductance can be minimized in order to achieve higher effective(actual) turns ratio.

Also, this transformer is optimized to operate at higher frequencies,for example, 500 kHz and above. Since there is no magnetic core,inductance of the winding of the coreless transformer is very small. Dueto this fact, higher frequency operation is necessary to achievereasonable usage, size and efficiency of the coreless transformer. Itcan be used in different ways:

a) To operate all the time, in which case it provides the necessary biasfor the circuitry on the output side of the converter; or

b) To operate only for predetermined periods of time during start-up orre-start after fault conditions such as over-current or over-voltageprotection, among others.

This mode is preferred, because of the low efficiency of the corelesstransformer caused by relatively large magnetizing current.

BRIEF DESCRIPTION OF THE DRAWING

The objects, advantages and features of the invention will be moreclearly perceived from the following detailed description, when read inconjunction with the accompanying drawing which illustrate by way ofexample the principles of the invention, in which:

FIG. 1 is a functional diagram of a bias circuit using a corelesstransformer with isolation, in accordance with an embodiment of theinvention;

FIG. 2 is a schematic diagram of an isolated dc-to-dc converter usingthe bias circuit with the coreless transformer of FIG. 1 for initialbias for control and drive circuitry referenced to the output of theconverter, in accordance with an embodiment of the invention;

FIG. 3 shows salient waveforms of the circuit of FIG. 1;

FIG. 4A is a partial schematic diagram of an alternative embodiment ofthe bias circuit of FIG. 1;

FIG. 4B is a partial schematic diagram of another alternative embodimentof the bias circuit of FIG. 1.

FIG. 5 is a partial schematic diagram of yet another alternativeembodiment of the bias circuit of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference now to the drawing and more particularly to FIG. 1, theinitial bias circuit according to an embodiment of the inventioncomprises oscillator 42, driver 43, coreless isolation transformer 58,rectifying diode 59 and capacitor 60. Oscillator 42 is controlled withENABLE input signal 102, usually generated by protection and controlcircuit 41 referenced to the input side of the converter. WithSTART/STOP signal 100 active, protection and control circuit 41generates ENABLE signal 102 coupled to oscillator 42. When signal 102 isactive (logic high, for example), oscillator 42 is enabled, andgenerates high frequency (for example, 500 kHz and above) output pulses101 of short duration. The frequency of pulses 101 is preferably atleast about 500 kHz with a duration of, for example, about 100nanoseconds. The short pulses 101 are fed into driver 43 which, in turn,drives coreless isolation transformer 58 via primary winding N_(P)referenced to the input side of the converter. The pulses from secondarywinding N_(S), being referenced to the output side of the converter, arerectified by diode 59 and fed into capacitor 60 which is charged to thevoltage V₃ level of voltage V_(CCS) (the (E) curve of FIG. 3), at timet=t₁. The level V₃ of voltage V_(CCS) is chosen to be higher than thestart-up voltage of controller 602 and driver 601, respectively, in FIG.2, reference to output side of the converter.

An isolated dc-to-dc converter using the invention of FIG. 1 is shown inFIG. 2. The converter could be also ac-to-dc or dc-to-ac. A forwardconverter is used as an example, but the invention is not limited to anyparticular topology. The forward converter comprises primary powercontrollable switch 500, isolation power transformer 400, rectifiers 402and 403, output inductor 405 and output capacitor 404. Note thatsynchronous rectifiers, such as MOSFETS, could be used instead ofrectifying diodes 402 and 403. The start-up circuit, comprisingresistors 801, 803, transistor 802 (shown as a MOSFET, for example),Zener diode 804 and capacitor 805, is conventional. Diode 701 isconnected with one end to winding N₃ and with its other end to resistor702. These two components, together with winding N₃, provide biasvoltage for control circuit 602 on the output side of the converter,once the converter has started. Note that an additional winding, havingthe same function for providing a bias voltage as winding N₃ (shown inFIG. 2 as one possible solution), could be added either as a separatewinding to isolation power transformer 400 or as a separate windingcoupled to output inductor 405, either of which is a very commonpractice.

For forward converter operation, when transistor 500 is on, a positivevoltage is applied across windings N₁ and N₃ of power isolationtransformer 400. Rectifier diode 402 is forward biased and current flowsinto inductor 405 and charges capacitor 404, supplying load 406. Whentransistor 500 is off, the voltages on windings N₁ and N₃ reversepolarity while the voltage on winding N₂ becomes positive andtransformer 400 is reset via forward bias diode 401. Here, the resetmethod is shown as an example only. It could be accomplished by anyother known means, including active reset. With winding N₃ havingreversed polarity, diode 402 is reverse biased, diode 403 is forwardbiased and inductor 405 discharges into capacitor 404 and load 406 viadiode 403.

The start-up circuit operates in the following manner. Capacitor 805 ischarged via transistor 802 and resistor 801 to a voltage equal to thedifference between the voltage of Zener diode 804 and the thresholdvoltage of transistor 802. Resistor 803 provides bias current for Zenerdiode 804 and transistor 802. The start-up circuit provides voltageV_(CCP), which supplies protection and control circuit 41 on the inputside of the converter, and also supplies the initial bias circuit whichcomprises oscillator 42, driver 43, coreless isolation transformer 58,diode 59 and capacitor 60.

Operation of the converter in FIG. 2 is initialized with START/STOPsignal 100 which activates protection and control circuit 41 which thengenerates ENABLE signal 102 to initiate oscillator 42. As describedabove, oscillator 42 generates narrow pulses with repetition rate T_(S),typically an order of magnitude longer than the pulse duration t_(p)(T_(S)>>t_(p) of pulse train (C) in FIG. 3), which are fed into driver43. Coreless isolation transformer 58 is driven by driver 43 with pulses103 similar to pulses 101. When a positive voltage pulse is appliedacross winding N_(P) of transformer 58 (the end of winding N_(P) markedwith a dot is positive with respect to input side return −V_(IN)), thevoltage on winding N_(S) is also positive (the end with a dot ispositive with respect to the other end) and diode 59 is forward biased.Capacitor 60 charges every time a positive voltage is applied acrosswindings N_(P) and N_(S) and after time t=t₁ reaches its maximum valueV₃. This value V₃ is chosen to be higher than the start-up voltage forcontroller 602 by proper choice of turns ratio N_(S)/N_(P), pulse widtht_(d) and period T_(S) of pulses 103, and voltage V_(CCP).

When ENABLE signal 102 is in active state, oscillator 42 is enabled andstarts generating pulses 101 for driver 43, which drives corelessisolation transformer 58. The relevant waveforms are shown in FIG. 3.Diode 59 rectifies positive pulses from secondary winding N_(S) oftransformer 58, and capacitor 60 charges to a predetermined voltage.Controller 602 is disabled until the voltage on capacitor 60, V_(CCS),reaches its start-up threshold (at time t=t₁). After that, controller602 starts operating and generates drive signal 603 for primary powerswitch 500 via, in this example, drive transformer 501. As soon ascontroller 602 starts operating, the voltage on capacitor 60 startsdropping until the voltage on winding N₃ is high enough so that diode701 becomes forward biased and charges capacitor 60 via current limitingresistor 702. The voltage on capacitor 60 drops until it reaches itssteady state value V₄ at time t=t₂, determined by the amplitude of thevoltage on winding N₃ minus the forward voltage drop across diode 701and the voltage drop across resistor 702. In one embodiment oscillator42 is disabled after a predetermined time (t=t₃ in FIG. 3) after voltageV_(CCS) reaches its steady state value V₄, and bias voltage V_(CCS) forcontroller 602 and driver 601 is provided after this time only fromwinding N₃ of power isolation transformer 400. Note that during timeinterval t₃−t₂ bias voltage is provided from both coreless isolationtransformer 58 and winding N₃.

In another embodiment of invention as shown in FIG. 4A, the time atwhich oscillator 42 is disabled is determined from drive signal 502,based on the amplitude and width of positive pulses applied totransistor 500 (see FIG. 2). In this manner oscillator 42 is disabledbefore predetermined time t=t₃ very soon after controller 602 commencesoperating and generating drive signal 502, which may be in the form ofshort pulses. One possible circuit implementation is shown in FIG. 4A,where additional circuit 509, comprising diode 503, resistor 504,capacitor 505 and resistor 506, receives voltage pulses 502 from thegate of transistor 500. The voltage on capacitor 505 depends on theamplitude and duration of voltage pulses 502, the capacitance ofcapacitor 505, and resistance of resistors 504 and 506. The voltage oncapacitor 505 is compared with reference voltage V_(R) in comparator 507and, when the voltage on capacitor 505 exceeds reference voltage V_(R),comparator 507 generates logic low signal 510 on its output which is fedinto protection and control circuit 41 and oscillator 42 becomesdisabled. Note that even when the circuit of FIG. 4A is used, it isadvantageous to disable oscillator 42 after predetermined time t=t₃ ifcontroller 602, and consequently the converter, is not operating or thevoltage on winding N3 (FIG. 2) is not big enough to provide bias voltageV_(CCS). Such conditions could be, for example, if over-currentprotection is activated, in which case the converter may operate with avery small duty cycle and consequently very narrow voltage pulses 502will not trip comparator 507 (FIG. 4A) and narrow pulses on winding N₃(FIG. 2) will not be enough to provide the minimum voltage on capacitor60 needed for operation of controller 602.

It is very common in practice that in event of activating either some orall protection (such as short circuit, over-current, over-voltage andover-temperature, for example), a converter enters so-called hiccupmode. In this mode the converter tries to re-start with a predeterminedperiod of operation in the event the converter is automatically shutdown due to the existence of a protection condition. Protection andcontrol circuit 41 is designed to generate ENABLE signal 102 which willbe a pulse train rather than the single pulse waveform (B) of FIG. 3.For example, in the embodiment shown, the pulse duration is about 5 msecwith an inactive duration of about 95 msec, for a total pulse period ofabout 100 msec. The status of signal 510 from circuit 509 (shown in FIG.4A) determines if protection and control circuit 41 will generate ENABLEsignal 102 as a pulse train. Whenever the ENABLE signal is active,capacitor 60 will be charged to voltage level V₃, controller 602 will beenabled and the converter will attempt to start again. If the converterdoes not start, or if it shuts down again due to a protection condition,circuit 509 detects that there is no drive signal 502 for transistor 500(FIG. 2) and generates logic low signal 510 which initiates an inactiveperiod in protection and control circuit 41. Oscillator 42 will beinactive for the remaining 95 msec. At the end of the inactive period,control and protection circuit 41 generates logic high ENABLE signal 102and the converter tries to re-start. It is also possible by using thedescribed embodiment to have on/off control referenced to the outputside of the converter. Note that the duration of active and inactiveperiods are given as examples only, and can be adjusted according to anyparticular application.

In still another embodiment of invention, shown in FIG. 4B, theconverter is enabled with ON/OFF signal 660 referenced to the outputside of the converter. Protection circuit 900 enables/disablescontroller 602 with signal 650. In order to have on/off control from theoutput side, START/STOP signal 100 is active, thus enabling protectionand control circuit 41 which generates ENABLE signal 102 as a pulsetrain rather than as a single pulse waveform, as described above in caseof the hiccup mode of operation. Note that the initial bias circuit alsoprovides voltage V_(CCS) for protection circuit 900. When ON/OFF signal660 becomes active and controller 602 is enabled, the converter entersits normal mode of operation as described above. Note that the inactiveperiod of ENABLE signal 102 determines maximum turn-on time of theconverter.

It is additionally advantageous in isolated converters to provide anenable/disable signal referenced to the input side of the converter thatinitiates or disables controller 602 referenced to the output ofconverter, for example in case of input voltage under- and over-voltageprotections or turning-on or turning-off the converter, as illustratedby FIG. 5.

With reference now to the circuit of FIG. 5, when protection and controlcircuit 41 is enabled, oscillator 42 is enabled and the bias circuitoperates as described above, but now continuously. After the initialtime t=t₃, the frequency of oscillator 42 is changed, for example, itmay be reduced, or supply voltage V_(CC) (which is different thanV_(CCP)) for driver 43 can be reduced, or both simultaneously, in orderto minimize power consumption while still providing pulses on secondarywindings N_(S) of coreless isolation transformer 58. By detectingpositive pulses from winding N_(S) with the peak detector circuitcomprising diode 750, capacitor 752 and resistor 754, the output sidecontrol circuit gets information that the module is enabled from theinput side.

The time constant defined by capacitor 752 and resistor 754 is chosensuch that the voltage across capacitor 752 decays in a predeterminedtime, which could be as low as the switching period of the converter.Comparator 865 senses the voltage across capacitor 752 and disablescontroller 602 (FIG. 2) whenever the sensed voltage is below V_(ref). Asmaller time constant will provide a shorter delay of disablingcontroller 602. Note that as long as oscillator 42 is enabled, there isvoltage across capacitor 752 that is higher than V_(REF), controller 602is enabled and consequently the converter is enabled from the inputside. Once the voltage across capacitor 752 drops below thresholdvoltage V_(REF), comparator 865 generates a disable signal forcontroller 602. In this manner, an on/off feature referenced to theinput side is sensed on the output side by the disabling of controller602. By disabling oscillator 42 on the input side, and sensing thevoltage drop on capacitor 752 on the output side, the on/off function istransferred from the input side to the controller on the output side ofthe converter.

It should be understood that the foregoing embodiments are exemplary forthe purpose of teaching the inventive aspects of the present inventionthat are covered solely by the appended claims and encompass allvariations not regarded as a departure from the intent and scope of theinvention. All such modifications as would be obvious to one of ordinaryskill in the art are intended to be included within the scope of thefollowing claims and their equivalents.

What is claimed is:
 1. A bias circuit used in a switch-mode powerconverters having an input and an output side, the bias circuitproviding initial bias power to a control circuit located on the outputside of the switch-mode power converter, the bias circuit comprising: anisolated coreless transformer, the transformer having windings, thewindings formed in or on layers of a printed circuit board (PCB); adriver coupled to the transformer, wherein the driver, when enabled byan enable signal, drives the coreless transformer with high frequencypulses of short duration; a rectifier coupled to the output of thetransformer to provide pulses; a capacitor coupled to the rectifier tosmooth the rectified pulses into a voltage, and providing the power tothe control circuit located on the output side of said of the converterduring startup.
 2. The bias circuit recited in claim 1, wherein saiddriver comprises: an oscillator that generates high frequency pulses ofshort duration, wherein the driver, when enabled, drives said corelesstransformer in response to the output of said oscillator.
 3. The biascircuit recited in claim 1, and further comprising an ON/OFF feature,wherein said ON/OFF feature comprises apparatus to disable said corelesstransformer and thus the control circuit located on the output side inresponse to a signal initiated on the input side.
 4. The bias circuitrecited in claim 1, and further comprising apparatus to disable saidcoreless transformer and thus said control circuit in response to asignal initiated on the output side.
 5. The bias circuit recited inclaim 1, wherein the enable signal is a single pulse of predeterminedduration.
 6. The bias circuit recited in claim 1, wherein the enablesignal is a train of pulses of predetermined duration and predeterminedperiod.
 7. The bias circuit recited in claim 6, wherein thepredetermined duration of each said pulse in the enable signal is shortcompared with the period of the pulses.
 8. The bias circuit recited inclaim 1, wherein the enable signal is a single pulse, the duration ofwhich is determined by the time from the commencement of the enablesignal until said converter commences operating.
 9. The bias circuitrecited in claim 8, wherein the enable signal has a predeterminedinactive period following the duration of the single pulse.
 10. The biascircuit recited in claim 2, wherein said oscillator operatescontinuously at a reduced frequency after a predetermined time.
 11. Thebias circuit recited in claim 2, wherein said oscillator operatescontinuously at a reduced frequency after said converter commencesoperating.
 12. The bias circuit recited in claim 1, wherein the voltageis reduced after a predetermined time.
 13. The bias circuit recited inclaim 1, wherein the voltage is reduced after said converter commencesoperating.
 14. The bias circuit recited in claim 1, and furthercomprising a sensor on the output of said coreless transformer, saidsensor disabling said control circuit pursuant to sensing that saidcoreless transformer has no output.
 15. The bias circuit recited inclaim 1, and further comprising a sensing and control circuit coupled tothe input side of said bias circuit to detect when said converter is notoperating, said sensing and control circuit commencing a short activeperiod where the control circuit located on the output side is enabledfollowed by a relatively long inactive period.
 16. A bias circuit usedin switch-mode power converters having an input and an output side, thebias circuit providing initial power to a control circuit located on theoutput side of the switch-mode power converter, the bias circuitcomprising: an isolated coreless transformer, the transformer havingwindings, the windings formed on layers of a printed circuit board(PCB), wherein the driver, when enabled by an enable signal, drives thecoreless transformer with high frequency pulses of short duration;driving means for driving said coreless transformer; rectifier means forproviding pulses at the output of said coreless transformer; capacitormeans for smoothing the rectified, the smoothed rectified pulsesgenerating a voltage for powering said control circuit located on theoutput side of said switch-mode power converter at startup.
 17. The biascircuit recited in claim 16, wherein said driving means comprises:oscillator means that generates high frequency pulses of short duration,the driving means for driving said coreless transformer in response tothe output of said oscillator.
 18. The bias circuit recited in claim 16,and further comprising an ON/OFF feature, wherein said ON/OFF featurecomprises apparatus to disable said coreless transformer and thus thecontrol circuit located on the output side in response to a signalinitiated on the input side.
 19. The bias circuit recited in claim 16,and further comprising apparatus to disable said coreless transformerand thus said control circuit in response to a signal initiated on theoutput side.
 20. The bias circuit recited in claim 16, wherein theenable signal is a single pulse of predetermined duration.
 21. The biascircuit recited in claim 16, wherein the enable signal is a train ofpulses of predetermined duration and predetermined period.
 22. The biascircuit recited in claim 21, wherein the predetermined duration of eachsaid pulse in the enable signal is short compared with the period of thepulses.
 23. The bias circuit recited in claim 16, wherein the enablesignal is a single pulse, the duration of which is determined by thetime from the commencement of the enable signal until said convertercommences operating.
 24. The bias circuit recited in claim 23, whereinthe enable signal has a predetermined inactive period following theduration of the single pulse.
 25. The bias circuit recited in claim 17,wherein said oscillator operates continuously at a reduced frequencyafter a predetermined time.
 26. The bias circuit recited in claim 17,wherein said oscillator operates continuously at a reduced frequencyafter said converter commences operating.
 27. The bias circuit recitedin claim 16, wherein the voltage is reduced after a predetermined time.28. The bias circuit recited in claim 16, wherein the voltage is reducedafter said converter commences operating.
 29. A method to provide aninitial bias and an enable signal for a control circuit (controller) onthe output side of a power converter having an input side, by use of acoreless transformer, the method comprising: providing the corelesstransformer fabricated in or on the layers of a printed circuit board;providing the controller, the controller controlled from output side ofthe converter; providing a controller transformer to transmit controlpower to power the input (primary) side of the power converter; drivingthe coreless transformer using high frequency pulses of short duration;and powering the controller using the coreless transformer.
 30. A biascircuit used in a switch-mode power converter having an input and anoutput side, the bias circuit providing an initial bias and an enablesignal for a control circuit (controller) located on the output side ofthe switch-mode power converter, the bias circuit comprising: anisolated coreless transformer, the transformer having windings formed inor on layers of a printed circuit board (PCB); a driver coupled to thetransformer to drive said coreless transformer, the driver comprising anoscillator that generates high frequency pulses, the driver to drive thecareless transformer in response to the output of the oscillator, togenerate pulses of predetermined duration followed by a relatively longinactive period; a rectifier coupled to the rectifier to providepositive pulses at the output of said coreless transformer; a capacitorcoupled to the rectifier to smooth the rectified positive pulses intothe enable signal, and providing the smoothed rectified pulses as theenable signal to generate a voltage to enable the controller located onthe output side of said switch-mode power converter.
 31. A bias circuitused in switch-mode power converters having an input and an output side,the bias circuit providing an initial bias and an enable signal for acontrol circuit (controller) located on the output side of theswitch-mode power converter, the bias circuit comprising: an isolatedcoreless transformer, the transformer having windings formed in or onlayers of a printed circuit board (PCB); driving means for generatinghigh frequency pulses of predetermined duration followed by a relativelylong inactive period for driving the coreless transformer, the drivingmeans comprising an oscillator means for generating the pulses, whereinthe pulses are of short duration; rectifier means for providing positivepulses at the output of said coreless transformer; capacitor means forsmoothing the rectified positive pulses, the smoothed rectified pulsesfor enabling the controller located on the output side of saidswitch-mode power converter.